Transparency sheets for image projection usually comprise a transparent plastic film as the support, with a monochrome or color image formed by toner fusion carried as an image on one surface thereof. The toner is generally composed of a binder resin such as polyester resin, a coloring agent (dye or pigment) and a static control agent. The toner is usually used to form a toner image with an electrophotographic system.
In the past, several problems have arisen when transparency sheets are fabricated by forming color images on overhead projector (OHP) transparent films with color copiers or printers employing electrophotographic systems. One of these problems has been insufficient melting of the color toner in the transparent film, resulting in residual uneven sections in the toner image layer. When uneven sections are present in the toner image layer it is difficult to avoid scattering of transmitted light, and this causes considerable reduction in the quality of the projected color image at the stage in which the transparency sheet is actually used in an OHP.
One known method of preventing undesirable light scattering on the surface of the toner image layer is to cover the toner image layer of the transparency sheet with a transparent film. That is, it has been proposed to cover the toner image layer with a transparent film so that the uneven sections of the toner image layer are buried and flattened by the transparent adhesive of the film and scattering of transmitted light is thereby reduced.
More specifically, Japanese Unexamined Patent Publication (Kokai) No. 2-38090, for example, discloses an image covering method characterized in that, after a toner image is formed on the transparent film, a cover sheet comprising a film and/or paper/release agent layer/thermoplastic transparent resin layer is combined with the toner image side of the film from the thermoplastic transparent resin layer side of the cover sheet, and the laminate is subjected to heating and pressure on a roller or plate, the thermoplastic transparent resin is cooled, and then the film and/or paper is released.
With this image covering method and other film covering methods, however, during the hand lamination when the covering sheet or film is manually combined with the color image side of an OHP transparent film, air becomes entrapped between the transparent film and the covering sheet or film, creating a new problem of residual air bubbles. Once air bubbles have been trapped, they are difficult to remove even using a tool such as a squeegee. When air bubbles reside on the color image side of the transparency sheet, they not only reduce the adhesion performance but also cast their shadows which are reproduced as image defects in the projected image, thus impairing the quality of the projected image.
Moreover, a high level of skill can be required for hand laminating covering sheets or films for OHP transparent films. In the case of conventional hand lamination, strong adhesive force is present from the adhesive layer when the covering sheet or film is attached to the OHP transparent film, and the resulting difficulty in achieving adequate positioning often leads to attachment in the wrong position. Reattachment after attachment in the wrong position is very difficult in most cases, and can damage the image sides of transparent films. Thus, there has been a need to provide covering sheets or films, particularly for OHP transparent films, that can be easily and precisely positioned and that can also be easily reattached after attachment in a wrong position.
Referring again to the image covering method described in Japanese Unexamined Patent Publication (Kokai) No. 2-38090, this method requires heat and pressure treatment using special means during lamination. The operation is complicated, the production costs are likely increased, and other problems also occur such as heat deformation of the transparent film or covering sheet during heating.
Difficult storage and filing has been another problem associated with conventional OHP transparency sheets. For example, when prepared transparency sheets are compiled in a binder or the like for storage, it has been necessary to perforate the edges of the transparency sheets, insert each of the transparency sheets into a specialized perforated holder and the close the binder.